Expected Loss to be Paid - Net Expected Loss to be Paid After Net Expected Recoveries for Breaches of R&W Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W	$ 660	$ 963	$ 737	$ 1,183
Economic Loss Development / (Benefit)	34	(37)	31	(39)
Accretion of discount	2	6	6	14
Changes in discount rates	1	(1)	32	(5)
Changes in timing and assumptions	31	(42)	(7)	(48)
Net (paid) recovered losses	41	34	(33)	(184)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	735	960	$ 735	960
Period after the end of the reporting period within which the ceded paid losses are typically settled (in days)			45 days
Loss and LAE Reserve paid	6	9	$ 9	16
Expected LAE to be paid	24		24		$ 33
Public Finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W	519	697	554	864
Economic Loss Development / (Benefit)	32	84	91	145
Net (paid) recovered losses	21	(9)	(73)	(237)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	572	772	572	772
Public Finance [Member] | United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W	493	666	531	832
Economic Loss Development / (Benefit)	30	92	86	154
Net (paid) recovered losses	20	(9)	(74)	(237)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	543	749	543	749
Public Finance [Member] | Non United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W	26	31	23	32
Economic Loss Development / (Benefit)	2	(8)	5	(9)
Net (paid) recovered losses	1	0	1	0
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	29	23	29	23
RMBS [Member] | United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W	104	237	146	293
Economic Loss Development / (Benefit)	1	(118)	(62)	(183)
Net (paid) recovered losses	23	43	44	52
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	128	162	128	162
RMBS [Member] | First Lien [Member] | United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W			166
Economic Loss Development / (Benefit)	(4)	19	55	50
Changes in discount rates	0		(25)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	122		122
RMBS [Member] | Second Lien [Member] | United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W			20
Economic Loss Development / (Benefit)	3	99	7	133
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	6		6
Other structured finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W	37	29	37	26
Economic Loss Development / (Benefit)	1	(3)	2	(1)
Net (paid) recovered losses	(3)	0	(4)	1
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	35	26	35	26
Structured Finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W	141	266	183	319
Economic Loss Development / (Benefit)	2	(121)	(60)	(184)
Net (paid) recovered losses	20	43	40	53
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	$ 163	$ 188	$ 163	$ 188